1. Field of the Invention
This invention relates to the remote sensing of pressure drops across an orifice plate in a pipeline and particularly to a stabilized tap mounting assembly to facilitate, stabilize and reduce vibration of the transmitter and associated apparatus, such as valved manifold blocks connecting the transmitter to taps on an orifice plate or orifice fitting.
2. Description of the Related Art
The gas industry makes extensive use of orifice plates for volume measurements. Orifice plates are artificial constrictions in a pipeline. As a result of this constriction, there is a pressure drop from the upstream side of the orifice plate to the downstream side of the orifice plate. There is a relationship between the pressure drop and the amount of gas flowing through the pipeline. The pressure drop can be measured, which in turn allows a determination of the flow through the pipeline.
The pressure drop is measured using electronic transmitters. These transmitters actually measure the pressures on both the upstream and downstream sides of the orifice plate and then record the difference between the two. Devices of this type are referred as differential pressure transmitters. There are numerous manufacturers of this type of device (e.g., Rosemount, Honeywell, Foxboro and others).
Orifice plate fittings (such as the type manufactured by Daniel Industries) and orifice flanges (manufactured by Daniel and others) are provided with holes (called taps) both upstream and downstream of the orifice plate. The taps are generally threaded holes, typically ½″ female NPT (National Pipe Thread—a tapered thread designed to seal pressure tight).
It is necessary to incorporate valving systems between the orifice taps and the differential pressure transmitters. These valving systems need to include the following functions. Block valves to isolate the flow through the upstream and downstream taps (generally referred to as the high pressure side and the low pressure side), between the orifice flange or fitting and the transmitter. An equalizer valve, or valves, to control the connection between the high pressure passages and the low pressure passages. A vent valve, or valves, to allow trapped pressures to be bled off or to allow the passages to be purged of entrapped gases or liquids. Sometimes the vent valve(s) are incorporated into the transmitters.
The natural gas industry has evolved a number of specifications and requirements to minimize errors in the measurement of flow and to provide improved accuracy and response time. Some of these are:                1. The closer the transmitter is to the orifice plate the better.        2. The orifice size through the valving system from the orifice taps to the transmitter must be ⅜″ in diameter.        3. The flow passages between the orifice taps to the transmitter should be as straight as possible. The ideal is a straight through passage. One 90 degree turn in the flow passage is permitted.        
Another consideration is mounting the valve system and the transmitters to the orifice plate assembly. Conventional mounting, valving, and transmitter assemblies can weigh 40 pounds and, depending on the complexity of the assembly and the products used, up to 70 pounds. The pipeline system and the orifice flanges or fittings are subject to vibration. The valving and transmitter assemblies also tend to be leaned on by the instrumentation personnel.
Thus, there is a continuing need to provide an alternative connection method and apparatus to operatively fluidly couple the high and lower pressure taps about an orifice plate with a transmitter.